1. Field of the Invention
The present invention relates to image forming apparatuses, image forming processes, and process cartridges for image forming apparatuses, which are adapted for lowering friction coefficients and lowering surface energies of electrophotographic photoconductors in particular.
2. Description of the Related Art
In the area of electrophotographic processes, various types have been proposed previously (see U.S. Pat. No. 2,297,691, Japanese Patent Application Publication (JP-B) No. 49-23910, and JP-B No. 43-24748, for example). Usually, in electrophotographic processes, electrostatic latent images are formed on photoconductors by various means along with making use of photoconductive substances, the latent images are developed using toner, the toner images are optionally transferred on paper, then the images are fixed by means of heating, pressing, solvent vaporization or the like, and images are formed.
The processes for developing electrostatic latent images are typically classified into liquid-developing processes that utilize a developer, which is usually prepared by dispersing finely various pigments and dyes in insulating organic liquid, and into dry-developing processes such as cascade, magnetic brush, powder cloud processes that utilize a toner, which is usually prepared by preparing a colorant such as carbon black into natural or synthetic resin. Currently, the dry-developing processes are employed broadly.
Recently, fixing at lower temperatures and outputting at higher speed are demanded for saving energy, therefore, thermal energy applied to toner at fixing has been decreasing. The toners for fixing at lower temperatures have been conventionally improved as for the fixing ability at lower temperatures by employing resins or waxes having lower softening temperatures. However, such toners for fixing at lower temperatures are not thermal resistant in general; therefore, they often solidify due to heat from associated apparatuses or heat at the preservation condition, thus so-called blocking or offset is often induced. Accordingly, sufficient temperatures can hardly be assured for such toners; namely, toners for fixing at lower temperatures without such problems have not been attained yet even polyester resins are employed, which exhibit relatively superior thermal preservation ability while affording appropriate fixing ability at lower temperatures. As such, thermal preservation ability and hot offset resistance are nowadays demanded for toners while maintaining lower fixing-initiating temperatures, such demands have been considered as inconsistent previously.
In order to solve such problems, various trials have been conducted; typical trial for producing a toner is such that a polymer having a site reactive with an active-hydrogen-containing compound, a binder resin, a colorant, and a releasing agent are dispersed or dissolved into organic solvent, the solution or dispersion is dispersed into an aqueous medium containing resin fine particles, then allowing to react between the active-hydrogen-containing compound and the polymer having a site reactive with the compound, the organic solvent is removed during or after the reaction, and the reaction product is rinsed and dried; wherein at least two types of resin fine particles are employed, one type provides fixing ability at lower temperatures, another type provides fixing ability at higher temperatures.
Such a toner may satisfy the aforesaid demand at least in part; however, some problems still remain, for example, extraordinary or abnormal images generate due to contaminant deposition or filming, induced possibly by resin fine particles for fixing ability at lower temperatures that may slightly remain on photoconductors; transfer efficiency decreases at transferring toner images on the photoconductor to transferring media, when image forming apparatuses are operated under high temperatures or the temperatures inside apparatuses comes to higher due to continuous operating.
In order to prevent the contaminant deposition such as fused substance onto photoconductor surface, various ways for lowering friction coefficient and surface energy of photoconductor surface have been proposed. As for lowering of friction coefficient as well as lowering surface energy of photoconductor surface, image forming apparatuses are proposed and have been come in practice that have a mechanism to feed lubricant onto photoconductor surface, as described in Japanese Patent Application Laid-Open (JP-A) No. 56-142567, for example. However, such an additional mechanism inevitably leads to enlarged or complicated apparatus, resulting in cost-up and complicated maintenance.
Further, addition of lubricant to surface layer is also proposed for lowering of friction coefficient and surface energy of photoconductor surface as described in JP-A Nos. 52-117134, 53-107841, 54-26740, 54-27434, 54-86340, 54-143142, 54-143148, 56-99345, 56-126838, 57-14845, 57-74748, 57-35863, 57-76553, 57-201240, 58-44444, 58-70229, 58-102949, 58-162958, 59-197042, 62-272281, 62-272282, 63-30850, 63-56658, 63-58352, 63-58450, 63-61255, 63-61256, 63-65449, 63-65450, 63-65451, 63-73267, 63-221355, 63-249152, and 63-311356, for example.
Examples of the lubricants include fluorine-containing resin (hereinafter “fluoropolymer”) such as polytetrafluoroethylene, spherical particles of acrylic resin, polyethylene resin powder, metal oxide powders such as silicon oxide and aluminum oxide, and liquid of silicone oil. Fluoropolymers with higher content of fluorine may provide a remarkable effect as lubricant due to their lower surface energy. The fluoropolymers are employed as crystalline fine particles, and formed into a surface layer or protective layer of photoconductors, after being dispersed into a binder resin such as acrylic resins, polyester resins, polyurethane resins, and polycarbonate resins.
However, when the content of fluoropolymer fine particles is relatively low, the friction coefficient tends to rise gradually under repeated image forming, even though the friction coefficient at the photoconductor surface may be decreased initially. On the other hand, the fluoropolymer fine particles tend to flocculate in the dispersion due to the inherent property of fluoropolymer, thus the uniform dispersion may be hardly prepared.
With respect to dispersing fluoropolymer fine particles, numerous investigations have been conducted and variously proposed (see JP-A Nos. 5-45920, 5-265243, 6-130711, 6-332219, and 8-87125, for example).
However, the surface layers that contain fluoropolymer fine particles disclosed in these references are mostly of lower content of fluoropolymer, thus the lower friction coefficient and lower surface energy may not possibly be maintained for long duration. JP-A No. 6-130711 listed above discloses higher content of the fluoropolymer, but any descriptions appear in terms of particle size after dispersion; therefore, finely dispersing seems to be difficult due to the flocculating tendency induced by higher content of fine particles of the fluoropolymer. Coated films formed from such coating liquid often result in excessive irregularity and localized existence of fluoropolymer fine particles on the coated surface. Excessive irregularity of the coated surface often leads to inferior cleaning or deteriorated toner images. Further, the localized fluoropolymer fine particles tend to yield micro areas with higher friction coefficient and higher surface energy and micro areas with lower friction coefficient and lower surface energy at the surface of photoconductors, which often causes inferior cleaning or deteriorated toner images. Moreover, excessively large secondary flocculate size of fluoropolymer fine particles may lead to scattering of laser light on the flocculates, which often causes turbulence of exposed latent images or insufficient potential contrast due to insufficient light amount, resulting in extraordinary or abnormal images.
Further, in the photoconductors that contains fluoropolymer fine particles at the surface region, there is still room in the continuous process in that occurrences of extraordinary images are not completely eliminated such as reduced resolution, void of halftone, and enlarged thin lines, which are estimated to result from lowered resistance of photoconductor surface by fluoropolymer fine particles. For the countermeasure, anti-oxidant agent may be added (see JP-A No. 8-292585 for example); however, a small amount of additive does not exhibit sufficient effect, whereas a large amount of additive may deteriorate electric properties. Further, incorporation of polyalylate resin is proposed as binder resin of the surface layer (see JP-A No. 8-248666 for example); however, conventional mono-component polyalylate resins cannot provide a sufficient effect currently. Accordingly, effective solution against the generation of extraordinary or abnormal images in continuous usage has not been found yet, as such.